This invention relates to certain monocyclopentadienyl metal compounds, to a catalyst system comprising a monocyclopentadienyl metal compound and an activator, and to a process using such catalyst system for the production of polyolefins, particularly ethylene-xcex1-olefin copolymers having a high molecular weight and high level of xcex1-olefin incorporation.
This invention relates to the discovery of various catalyst ligand structure affects which are reflected in the activity of the catalyst system and in the physical and chemical properties possessed by a polymer produced with a monocyclopentadienyl catalyst system. Accordingly, various species within the general class of monocyclopentadienyl metal catalysts have been discovered to be vastly superior in terms of the ability of such species to produce ethylene-xcex1-olefin copolymers of high molecular weight with high levels of xcex1-olefin comonomer incorporation and at high levels of catalyst productivity.
As is well known, various processes and catalysts exist for homopolymerization or copolymerization of olefins. For many applications it is of primary importance for a polyolefin to have a high weight average molecular weight while having a relatively narrow molecular weight distribution. A high weight average molecular weight, when accompanied by a narrow molecular weight distribution, provides a polyolefin or an ethylene-xcex1-olefin copolymer with high strength properties.
Traditional Ziegler-Natta catalyst systemsxe2x80x94a transition metal compound cocatalyzed by an aluminum alkylxe2x80x94are capable of producing polyolefins having a high molecular weight but a broad molecular weight distribution.
More recently a catalyst system has been developed wherein the transition metal compound has two or more cyclopentadienyl ring ligandsxe2x80x94such transition metal compound being referred to as a metallocenexe2x80x94which catalyzes the production of olefin monomers to polyolefins. Accordingly, metallocene compounds of a Group IV B metal, particularly, titanocenes and zirconocenes, have been utilized as the transition metal component in such xe2x80x9cmetallocenexe2x80x9d containing catalyst systems for the production of polyolefins and ethylene-xcex1-olefin copolymers. When such metallocenes are cocatalyzed with an aluminum alkylxe2x80x94as is the case with a traditional type Ziegler-Natta catalyst systemxe2x80x94the catalytic activity of such metallocene catalyst system is generally too low to be of any commercial interest.
It has since become known that such metallocenes may be cocatalyzed with an alumoxanexe2x80x94rather than an aluminum alkylxe2x80x94to provide a metallocene catalyst system of high activity for the production of polyolefins.
The zirconium metallocene species, as cocatalyzed or activated with an alumoxane, are commonly more active than their hafnium or titanium analogues for the polymerization of ethylene alone or together with an xcex1-olefin comonomer. When employed in a non-supported formxe2x80x94i.e., as a homogeneous or soluble catalyst systemxe2x80x94to obtain a satisfactory rate of productivity even with the most active zirconocene species typically requires the use of a quantity of alumoxane activator sufficient to provide an aluminum atom to transition metal atom ratio (A1:TM) of at least greater than 1000:1; often greater than 5000:1, and frequently on the order of 10,000:1. Such quantities of alumoxane impart to a polymer produced with such catalyst system an undesirable content of catalyst metal residue, i.e., an undesirable xe2x80x9cashxe2x80x9d content (the nonvolatile metal content). In high pressure polymerization procedures using soluble catalyst systems wherein the reactor pressure exceeds about 500 bar only the zirconocene or hafnocene species may be used. Titanocene species are generally unstable at such high pressure unless deposited upon a catalyst support.
A wide variety of Group IV B transition metal compounds have been named as possible candidates for an alumoxane cocatalyzed catalyst system. Although bis(cyclopentadienyl) Group IV B transition metal compounds have been the most preferred and heavily investigated for use in alumoxane activated catalyst systems for polyolefin production, suggestions have appeared that mono and tris(cyclopentadienyl) transition metal compounds may also be useful. See, for example U.S. Pat. Nos. 4,522,982; 4,530,914 and 4,701,431. Such mono(cyclopentadienyl) transition metal compounds as have heretofore been suggested as candidates for an alumoxane activated catalyst system are mono(cyclopentadienyl) transition metal trihalides and trialkyls.
International Publication No. WO 87/03887 describes the use of a composition comprising a transition metal coordinated to at least one cyclopentadienyl and at least one heteroatom ligand as a transition metal component for use in an alumoxane activated catalyst system for xcex1-olefin polymerization. The composition is broadly defined as a transition metal, preferably of Group IV B of the Periodic Table, which is coordinated with at least one cyclopentadienyl ligand and one to three heteroatom ligands, the balance of the transition metal coordination requirement being satisfied with cyclopentadienyl or hydrocarbyl ligands. Catalyst systems described by this reference are illustrated solely with reference to transition metal compounds which are metallocenes, i.e., bis(cyclopentadienyl) Group IV B transition metal compounds.
At the Third Chemical Congress of North American held in Toronto, Canada in June 1988, John Bercaw reported upon efforts to use a compound of a Group III B transition metal coordinated to a single cyclopentadienyl heteroatom bridged ligand as a catalyst system for the polymerization of olefins. Although some catalytic activity was observed under the conditions employed, the degree of activity and the properties observed in the resulting polymer product were discouraging of a belief that such mono(cyclopentadienyl) transition metal compound could be usefully employed for commercial polymerization processes.
Although the metallocene/alumoxane catalyst system constituted an improvement relative to a traditional Ziegler-Natta catalyst system, a need existed for discovering catalyst systems that permit the production of higher molecular weight polyolefins and desirably with a narrow molecular weight distribution. Further desired was a catalyst which, within reasonable ranges of ethylene to xcex1-olefin monomer ratios, will catalyze the incorporation of higher contents of xcex1-olefin comonomers in the production of ethylene-xcex1-olefins copolymers.
EPO 416,815 discloses certain mono(cyclopentadienyl) metal compounds which are activated with an alumoxane cocatalyst. U.S. Pat. No. 5,064,802 discloses certain mono(cyclopentadienyl) metal compounds which are activated with a non-coordinating compatible anion of a Bronsted acid salt.
The present invention is a catalyst system including a mono(cyclopentadienyl) metal compound and an activator component. The catalyst system is highly productive for polymerizing ethylene and xcex1-olefins to produce a high molecular weight ethylene-xcex1-olefin copolymer having a high content of xcex1-olefin. More particularly, the present invention relates to certain mono(cyclopentadienyl) metal compounds which include an amido moiety having an aliphatic or alicyclic hydrocarbyl group covalently bonded thereto through a primary or secondary carbon atom.